nFeed field to nFeed field (same nfeed keys)You can configure this in the corresponding script fields by adding the following linesData Filter

replica.customFields."nfeed field" = issue.customFields."nfeed field"

Change Processor:

issue.customFields."nfeed field" = replica.customFields."nfeed field"

nFeed field to nFeed field (different nfeed keys)As Francis answered and documented in their docs. The implementation depends on the kind of datasource of your nfeed field (database table, csv/JSON file, etc.).

nFeed field to text fieldGet the text value from the nfeed field and write it into a text field. You can use the first part of this doc (Looking up nfeed values) in order to send out the value on the senders side. On the receiver side you have to map it to the corresponding text field.

With exalate you can create a script which retrieves the nfeed information through REST on one side, transport it over the wire to the other JIRA, and lookup the corresponding value in the database table.